1. Field of Invention
The invention relates to a structure of inkjet head and, in particular, to a connection structure of a stacked inkjet head.
2. Related Art
The main technologies involved in inkjet print heads are piezoelectric inkjet heads and thermal bubble inkjet heads. The difference between them is whether the actuator used for pushing ink is of the thermal bubble type of the piezoelectric type. The thermal bubble actuator uses a heater to instantaneously vaporize ink, producing high-pressure bubbles to push ink out of nozzles. The piezoelectric actuator uses deformation of piezoelectric ceramics under an external voltage to push liquid out of nozzles. Relative to the thermal bubble type, the piezoelectric inkjet head does not have chemical changes resulting from high temperatures to affect the printing quality. Moreover, it does not have repeated high thermal stress. Therefore, it is more durable.
Since the deformation of piezoelectric ceramic materials is not too large, the channel has to be specially designed in order to eject droplets. The conventional method of making piezoelectric inkjet heads normally take several pieces of machined plates and stack them together to obtain a special fluid structure. The machining of the plates is normally performed by wet etching. However, when the etching pattern changes significantly as one needs to make large-area channels and small-size nozzles, the etching speed may become unstable. This is the etching error. The reason is that the reaction ions for nozzles will be taken away by nearby large-area channels, resulting in a lower etching speed than others.
During the process of assembling many plates, they have to be accurately aligned. Since the piezoelectric material is ceramics, the plate junction can be achieved by stacking and sintering several layers of green sheets. For example, the method for making multiple layered inkjet head disclosed U.S. Pat. No. 6,134,761 stacks several layers of ceramics to form a fluid structure with an actuator, ink channels, and a cavity. The structure is sintered and combined with a nozzle plate and fluid structure by co-fired process. Nevertheless, the sintered ceramic green sheets may encounter precision problems as sintering shrink. Moreover, there may have cracks or bubbles when stacking the green sheets. This will cause problems in the strength of the fluid structure.
On the other hand, one often uses epoxy or solder for the connection of some plates. For example, the piezoelectric inkjet head described in the U.S. Pat. No. 5,598,196 has the cover plate and the fluid structure connected by soldering. The soldering metal also provides electrical communications with the exterior. However, the coating precision for connections using adhesive is very stringent; otherwise, it is likely to have such problems as cracks, departure or adhesive overflow to clog the channels or nozzles. Therefore, as disclosed in the U.S. Pat. No. 6,037,707, a connection structure for the electrodes of a piezoelectric ceramic actuator and a piezoelectric ceramic layer is used to enhance the connection among the plates. A rough surface is formed on the upper surface of the piezoelectric ceramic layer to increase the junction area. An adhesive is used to connect the upper surface of the piezoelectric ceramic layer and a deformable electrode. A similar principle can be applied to the connections of nozzle plates. As shown in the U.S. Pat. No. 5,855,713, micro cavities are formed on the surface of a nozzle plate by laser ablation. Then an adhesive is used to stick the nozzle plate to the fluid structure.